LIGO’s first black hole merger may have been 10 billion years in the making

Big, luminous stars might simply be blinking out of existence.

The LIGO detector has now seen at least two black hole mergers. The second merger it spotted was about what we would expect given a binary system of two massive stars. Both explode, leaving black holes behind that are just a bit more massive than the Sun; these later go on to merge.

Further Reading

But the first merger detected by LIGO was something rather unusual given that both black holes were around 30 times the Sun's mass. So far, we have not observed anything that could produce black holes in that mass range. Now, a new modeling study suggests that mergers with these sorts of masses might be common—but only if stars can collapse directly into a black hole without exploding first. This situation would require some of the Universe's most luminous stars to simply be winking out of existence.

Further Reading

The black holes involved in these mergers almost certainly began their existence as binary star systems. So in the new study, the authors performed a massive number of simulations of these systems using a modeling package called StarTrack. The simulations took into account the different amount of heavy elements present at different times in the Universe's existence—there are 32 different levels of heavy elements, and the team ran 20 million simulations at each of them. The simulations also took into account various models of the collapse of massive stars, as well as whether the process generated an asymmetrical force that could kick the resulting black hole into an energetic orbit.

With the simulations run, the authors could sift through the results and look for systems that produced the sorts of heavy black holes that LIGO detected merging. They could then play back the simulation and examine the process that produced the black holes in the first place.

The models indicate that the systems that produce LIGO-like mergers started out as giant stars with very few heavier elements. Giant stars have only 10 percent of the metal levels found in the Sun, but they're somewhere between 40 to 100 times more massive. These sorts of stars were much more common in the early Universe, and 75 percent of the simulations indicate that the binary system formed within the first two billion years of the Universe's existence.

In the simulations, helium and heavier elements form quickly at the core of these stars. The core then ejects its outer layer of hydrogen. This action creates a pair of Wolf-Rayet stars, extremely bright and compact objects.

At this point, one of the two stars does something unusual: it collapses directly into a black hole without exploding in a supernova. While there has been a lot of theoretical work that indicates it should be possible, we've never actually observed a star collapsing out of existence.

Assuming it happens, the resulting black hole should be in the right mass range to produce a LIGO-like merger. But the merger doesn't happen immediately (indeed, it may take 10 billion years after the birth of the stars). Because of the lack of explosion, the black hole would continue to orbit close to its companion. Over time, it would help draw out the outer layers of the companion star, creating a situation where the black hole's orbit would be inside the envelope of the star.

After the star contracts, the result would look like what we call X-ray binaries: an X-ray source orbiting a massive, luminous star. The authors point out that we've observed two systems that look like the modeled results (IC10 X-1 and NGC 300 X-1). Given a bit more time, however, the second star also undergoes a direct collapse, creating a pair of orbiting black holes, each about 30 times as massive as the Sun. These black holes then take about 5 billion years to spiral into each other and merge.

Similar binary systems are still forming, so the simulations also indicated that there was a 25-percent chance that an accelerated version of this process might have started within the last two billion years.

But really, the most notable thing about this process is the fact that it really only works through the direct collapse of stars. "A striking ramification of this is the prediction that hot and luminous Wolf–Rayet progenitors of massive black holes should disappear from the sky as a result of direct collapse to a black hole (that is, with no supernova explosion)," the authors write. Since these are the most luminous stars in the sky, that event should be easy to spot; the authors note that observational campaigns intended to do just that are already underway.

Until we observe this process happening, these simulations should be viewed as pretty tentative. They provide a possible explanation for one of the puzzling aspects of the LIGO observation, but the main appeal is that most of the other explanations are even less physically plausible.